1. Field of the Invention
The present invention relates to a sheet which may be used in firing ceramic faced base plates, metal faced base plates and glass faced base plates. More specifically, the present invention relates to a sheet which may be placed between adjacent stacked base plates and between the base plates and the supporting base during firing to prevent adhesion. The sheet additionally prevents the formation of undulations and pin-holes in the base plates during firing.
2. Description of the Prior Art
When ceramic faced base plates are fired according to the conventional method, a powder of material with a high melting point, such as Al.sub.2 O.sub.3 is scattered on the ceramic faced base plates by hand, following which ceramic faced base plates are stacked in a vertical direction and fired. However, due to the uneven scattering effect obtained by this method, undulations and pin-holes form in the ceramic faced base plates during firing. In addition, inferior products are often produced due to the adhesion of Al.sub.2 O.sub.3 powder to undesirable portions of the ceramic faced base plates. Furthermore, the method of scattering the powder on the base plates by hand increases the cost of production.
Japanese Patent Publication SHO No. 60-8991 proposes a sheet which comprises a powder of Al.sub.2 O.sub.3 or ZrO.sub.2 scattered within a paper or plastic sheet to overcome the above-described drawbacks in the method of scattering a the powder with a high melting point on the base plates by hand. However, such a sheet shrinks considerably during firing according to the type of the sheet used. As a result, the peripheral portions of the ceramic faced base plates often adhere to each other during and following firing. To prevent such adhesion, the sheet thus comprising the powder must be increased in size by the estimated amount of shrinkage expected during firing and this estimation is very difficult.
During firing, the stacked ceramic faced base plates are usually placed on a supporting base. Metal-faced base plates, which are constructed by pressing a powder of a metal such as cemented carbide into the form of a plate, may also be stacked and fired. During firing, the stacked metal-faced base plates are usually put on a supporting base, as in the case of the ceramic faced base plate. Since ceramics-faced base plates and metal-faced base plates shrink during firing, the bottom surface of the stacked base plates becomes scratched due to abrasion with the surface of the supporting base, thereby producing an inferior product. Even in the case of placing the sheet comprising a powder scattered within a paper, as disclosed by Japanses Patent Publication SHO No. 60-8991, between the stacked ceramic faced or metal faced base plates and the supporting base, the lubricating effect of said sheet is insufficient to prevent abrasion. In particular, almost no lubricating effect may be expected in the case of metal-faced base plates.